42 DAIRY BACTERIOLOGY 



in forming eyes in Emmental cheese, resembles the streptococci, 

 but it does not coagulate milk. To this group belong various colour- 

 producing species which may form red and brown spots in Emmen- 

 tal cheese 1 . Other strains (b) are stout irregular rods, some of 

 which become either branched or club-shaped, and stain so un- 

 evenly that, like the diphtheria bacteria, they present a segmented 

 appearance. These species produce sufficient acid (not lactic 

 acid) from milk sugar to coagulate milk. The propionic acid 

 bacteria do not decompose casein. Some of them are anaerobic, 

 but may be accustomed to aerobic conditions ; when this occurs, 

 their fermentative powers are gradually weakened. The aerobic 

 varieties form outstanding colonies in stab cultures, and are on 

 the whole as prone to slime production as the aerogenes type. 

 The propionic acid bacteria grow at temperatures between 15 and 

 40 C. According to Bum 2 , they occur in appreciable numbers in 

 cow dung, and probably they find their way into milk from this 

 source. 



THE BUTYRIC ACID FERMENTATION 



In this process carbohydrates or lactates are converted into a 

 number of different products, among which butyric acid, carbon 

 dioxide and hydrogen have attracted particular notice. In 

 addition to these, notable amounts of lactic, acetic, propionic and 

 formic acids are formed, and sometimes also various alcohols, so 

 that the process is an extremely complicated one. 



The first butyric acid bacterium was described by Pasteur in 

 1861, a discovery which for the first time revealed the existence of 

 obligate anaerobic organisms ; the butyric acid bacteria will under 

 no circumstances grow in presence of atmospheric oxygen. They 

 are fairly large rods which form spores, assuming either the 

 Clostridium or Plectridium forms (Figs. 5 and 6) ; the spores 

 exhibit polar germination. In the butyric acid bacteria it is often 

 possible to demonstrate reserve food material which stains blue or 

 violet with iodine ; this is most noticeable just before sporing if 

 grown on starchy material such as potato slices. The young 

 bacteria are generally Gram-positive. The true butyric acid 

 bacteria can subsist on inorganic nitrogen, and in conjunction 

 with aerobic bacteria they can assimilate atmospheric nitrogen 3 . 

 They do not attack proteins ; in this respect they differ from a 



1 Thoni and Allemann, " Centralblatt fiir Bacteriologie," 2 Abt., 1910, 

 Bd. XXV., p. 8. According to the author's investigations, stab cultures of 

 white varieties of the propionic acid bacteria are often red below the surface 

 of the medium. 



" Landwirtschaftliches Jahrbuch der Schweiz," 1912, p. 481. 



3 Winogradsky, " Archives des Sciences hiologiques," 1895, tome 3, 

 p. 295, and Bredemann, " Zentralblatt f. Bact.," 2 Abt., 1909, Bd. XXIIL, 

 p. 385. 



